There is no such thing as a ActivityPub node whereas there is such a thing as a Mastodon node. I joined a Mastodon node that uses the ActivityPub protocol, because the Mastodon node I joined uses this protocol, I can communicate with software nodes other than Mastodon that also use the ActivityPub protocol.

Maybe you are better off asking why people say Mastodon when they mean Fediverse. Even ActivityPub's user guide says to join a Mastodon node, they don't say to join a ActivityPub node.

https://socialhub.activitypub.rocks/pub/guide-for-activitypu...